The problem of keeping data transmitted from a sender to a receiver confidential against an adversary acting as an eavesdropper can be solved using encryption schemes. In a nutshell, a (symmetric) encryption scheme is a pair of algorithms: an encryption algorithm, run by the sender, that, on input a secret key and clear data, returns encrypted data; and a decryption algorithm, run by the receiver, that, on input a secret key and encrypted data, returns clear data. (See FIG. 1, described below, depicting an associated model.) The basic correctness requirement is that if the secret keys used by sender and receiver are the same, the clear data recovered by the receiver is precisely the one that was sent by the sender. The basic security (or, more precisely, confidentiality) requirement is, informally speaking, that if the secret key used by sender and receiver is random and unknown to the adversary, then the adversary obtains no information about the clear data from the encrypted data. Note that the adversary is given full access to the encryption and decryption algorithm (but no access to the associated secret key). Several stronger variations of this requirement are actually studied, where the adversary can mount more elaborated attacks, such as “chosen-message”, and “chosen-ciphertext” attacks. Classic encryption schemes, developed until the 20th century, where based on basic principles of “confusion” and “diffusion”. The first provable confidential scheme, the One-Time Pad, invented by Vernam in the early 1900's, and analyzed by Shannon in its pioneering works in the mid 1900's, was the first provable secure encryption scheme, but is today considered inefficient (as a stand-alone scheme) as it requires a number of random bits at least equal to the number of data bits. Modern encryption schemes use short (e.g., 128-bit) random keys, and are based on block ciphers (such as AES), composed using appropriate modes of operations (such as the CBC mode). Such schemes have limited provable confidentiality properties but are widely believed to be secure and are thus employed in all applications.
Communication over OCDM-based networks allows a receiver to obtain data from multiple senders or from a single sender using multiple parallel data streams. A public encoding algorithm is used by the sender to simultaneously process these data streams, and a public decoding algorithm is used by the receiver to decode any single one of the sender's data streams. The optical fiber physical conditions induce inter-code phase shifts on the data encoded by the sender, but such shifts are not changing the receiver's ability to obtain the sender's data. (See FIG. 2, described below, depicting an associated model.) When no encryption procedure is performed, just as with conventional networks, an adversary acting as an eavesdropper can use the same receiver's algorithm to decode data and thus violate data confidentiality.
Prior techniques for providing security for ultra high bandwidth optical communications over WDM networks includes the use of conventional electronic digital encryption which is not readily scalable to very high data rates and is not robust to archival attack and spoofing. Another prior technique is the use of Essex's phase scrambling of a single modulation broadened laser line which is not robust to known plain text (KPT) attack.
Optical code division multiplexing (OCDM)-based security by obscurity has been promoted as a scalable “security” solution for spectral-phase encoded OCDM systems operating at aggregate data rates of 100 Gb/s and beyond that can be realized with available technology through inverse multiplexing of 10 Gb/s tributaries, each carried on a OCDM code. Such a scheme is described in S. Etemad et. al., “OCDM-Based Photonic Layer “Security” Scalable to 100 Gb/s for Existing WDM Networks”, invited paper in the Journal of Optical Networking volume 6, issue 7, pages 948-976, July 2007. The approach is based on the early proposal that scrambling of the phase of the combined aggregate of OCDM codes in use increases the search space beyond the reach of an exhaustive search attack. See, R. Menendez et al., “Network Applications of Cascaded Passive Code Translation for WDM-Compatible Spectrally Phase Encoded Optical CDMA,” IEEE J. of Lightwave Technology, Vol. 23, pp. 3219-3231, 2005. The earlier solution has been demonstrated in the laboratory for an aggregate 40 Gb/s over 400 km transmission distance. See, P. Toliver et al., “40 Gb/s OCDM-based Signal Transmission over 400 km Using Integrated Micro-Ring Resonator-based Spectral Phase Encoding and Quaternary Code Scrambling for Enhanced Data Confidentiality”, ECOC2007, Post Deadline Paper 33. However, robustness against known plain text (KPT) attacks was questioned by showing with some idealized assumptions that the search space is dramatically reduced from pn to p(n-m), where n is the number of phase-locked wavelengths and also the maximum number codes available, (n-m) is the actual number of codes in use and p is the number phase states supported by the scrambler. See, S. Goldberg, et. al. “Towards a Cryptanalysis of Spectral-Phase Encoded OCDMA with Phase-Scrambling”, OFC 2007, OTH-J7.